Conveying devices, such as those which are used for moving objects between different stations in a factory, usually comprise a conveying track in the form of a belt or a chain. The conveying tracks can be recessed in a trench with vertical side surfaces. Alternatively, they can be located on the horizontal upper surfaces of the trench or arranged in some other way. The objects to be conveyed are arranged slidably in relation to the conveying tracks, either directly or via transportation means, which are also known as pallets.
A pallet is conveyed along the conveying device comprising different work stations. In some cases, every work station will perform one operation on the transported object. In those cases, the work flow is designed such that every pallet will stop at every work station. In other cases, for example when different products or different versions of the same product are produced in the same product flow, all pallets do not have to stop at all the work stations.
If the conveying device is of a conventional type, in which all work stations are positioned after each other and the operations on the objects are performed synchronous, every pallet have to stop at each work station. If no operation is to be performed on an object at a specific work station, the object will have to wait at that work station until the operation on the previous object at that work station is ready, until the object can continue. All pallets are in this case released at the same time and moves at the same time. One advantage of such a system is that it is easy to predict the performance of the system. One disadvantage of such a system is that it is not very flexible. If the operation time at a work station is long, all objects will have to wait that time, even if there is no operation performed on all objects or if the operation time at another work station is considerably shorter. Thus, such systems are mostly used for a single product or when the differences in the products are small.
In other systems, the operations on the objects are performed asynchronous, i.e. the pallet stops at a work station where an operation is to be performed, and is released when the operation is ready. In such systems, a relatively large buffer is required between each work station in order to compensate for the different through-flow times of the products and for the different operation times at a work station.
Another way of compensating for different operation times at a work station is to use two or more work stations that perform the same task. They are often positioned close to each other, and the object is stopped at the work station that is free. In this way, the through-flow time of the system can be balanced in order to avoid bottlenecks in the system.
Normally, the operator setting up the system will try to coordinate the work so that every pallet spends the same length of time at each work station. Sometimes, it is difficult to coordinate the operation time at each work station. One solution, apart from using a buffer, is to locate some work stations along different sub-conveyors located on the side of the main conveyor. Such a solution will require extra space and extra parts, such as a drive motor and means to divert objects to the sub-conveyor. This solution will work in some cases, but in other cases, especially when the product flow is altered after production has started, it may not be possible to rebuild the conveying system with such a solution. There is thus room for improvements.